mwe.tex     % instance of personal letter
address.tex % full address of individual being addressed in mwe.tex
C:LocalTeXtexlatexmymygeneralmatter.sty    % author's address and signature defined here
C:LocalTeXtexlatexmymyletter.cls   % author's address and signature defined here

NeedsTeXFormat{LaTeX2e}
documentclass{myletter}

newcommand{thetoaddress}{input{address}} % doesn't compile
%newcommand{thetoaddress}{%
%Addressee name\
%Addressee title\
%Address line 1\
%Address line 2\
%Address line 3%
%}  % does compile
newcommand{theopening}{Dear addressee,}
newcommand{theclosing}{Sincerely,}
newcommand{thepostscript}{}

begin{document}
I would like to thank you for your recommendation during the hiring process for the position of position with company.

I have thoroughly enjoyed being a part of company.  It has indeed been a privelege to have worked with such a competant team.

I very much appreciate the support.
end{document}

Person\
Title\
Address line 1\
Address line 2\
Address line 3

NeedsTeXFormat{LaTeX2e}
ProvidesPackage{mygeneralmatter}[2013/05/21 a package containing general matter]

newcommand{email}{first.last@domain.com}  % appropriate email address

newcommand{phone}{(999) 999 9999}     % phone number

% company name and address
newcommand{authoraddress}{%
Address line 1 \%
Address line 2  \%
Address line 3 %
}%

% company name, address and phone numbers
newcommand{authoraddresslong}{%
authoraddress \%
phone \%
email
}%

% signature for formal letters
newcommand{authorsignature}{First Last}

NeedsTeXFormat{LaTeX2e}
ProvidesClass{myletter}[2013/04/22 a class for a personal letter which essentially preloads the authors current personal information]
LoadClass[]{letter}

RequirePackage{mygeneralmatter}    % provides the commands: authorsignature, authoraddresslong

signature{authorsignature}    % signature of the letter defined in jclgenmatter.sty
address{authoraddresslong}    % author's long address defined in jclgenmatter.sty

AtBeginDocument{%
begin{letter}{thetoaddress}
opening{theopening}
}%

AtEndDocument{%
closing{theclosing}
ps{thepostscript}
end{letter}
}

! Argument of @no@pgbk has an extra }.
<inserted text>
                par
l.22 end{document}

RequirePackage{catchfile}
newcommand{getaddressfrom}[1]{CatchFileDef{thetoaddress}{#1}{}}
newcommand{defineaddress}[1]{defthetoaddress{#1}}

AtBeginDocument{%
  begingroupdeftempa{endgroupbegin{letter}}
    expandaftertempaexpandafter{thetoaddress}%
  opening{theopening}
}

documentclass{myletter}

getaddressfrom{address}

newcommand{theopening}{Dear addressee,}
newcommand{theclosing}{Sincerely,}
newcommand{thepostscript}{}

begin{document}
I would like to thank you for your recommendation during the hiring
process for the position of position with company.

I have thoroughly enjoyed being a part of company.  It has indeed
been a privelege to have worked with such a competant team.

I very much appreciate the support.
end{document}

defineaddress{John Doe\
  42, Some Street\
  12345 Somecity}

newcommand{thetoaddress}{protectinput{address}} % does compile

textheight=9in % text height can be bigger for a longer letter
textwidth=7in  % text width of `6.5in` leaves 1 inch for right margin